The present invention is directed to magnetic resonance imaging (MR imaging, or MRI) and more particularly to MR imaging in which water-only and fat-only images are taken without the interference of each other, and combined water-plus-fat images are formed.
MR images are the presentation of the specific chemical environment of the imaged protons. The information from the chemical environment of the protons can be acquired in many different ways. A pulse sequence is applied to excite the protons and to acquire the information. The pulse sequence is designed to acquire specific information from specific chemical species. The most useful species in living tissues are water and fat. Therefore, several pulse sequences have been developed to acquire and display information only from water containing tissues or only from fat containing tissues. The routine sequence displays data from both water and fat containing tissues. The water-only images have widespread use in the clinical environment due to their ability to eliminate the high signal intensity fat and highlight the pathology in the organs and structures. The disadvantages in the previously developed sequences have been related to inadequate elimination of the chemical species not imaged (fat in particular), to a chemical shift artifact which is related to the different precession frequency between water and fat resulting in local misplacement of the anatomic structures, and to long acquisition time. In addition, if both fat suppressed (water-only) images and non-fat-suppressed (water-plus-fat) images are needed, two different data acquisitions are required with an increased imaging time. The long acquisition time to acquire water-plus-fat and water-only images is specifically a disadvantage in 2D dual-echo spinxe2x80x94echo (SE) imaging where acquisition times are inherently long.
Fat suppression sequences have often been used clinically to improve musculoskeletal system evaluation. Among those sequences, T2-weighted (T2W) water-only (fat-suppressed) spin echo images are most commonly used in clinical practice. Such images are described in T. T. Miller et al, xe2x80x9cFat-suppressed MRI of musculoskeletal infection: fast T2-weighted techniques versus gadolinium-enhanced T1-weighted techniques,xe2x80x9d (Skeletal Radiol. 1997; 26:654-658). Although water-only images demonstrate the pathology, water-plus-fat (non-fat suppressed) proton density weighted (PDW) images are still needed for anatomic details. So far, water-only and water-plus-fat images are normally acquired using two separate imaging sequences. The increasing pressure in clinical practice to increase throughput, however, limits the number of sequences that can be prescribed in a clinical study, prompting the development of pulse sequences that will improve the ratio of information provided to the imaging time used.
Different techniques have been proposed to obtain water-only and fat-only images. In chemical saturation techniques an additional long spectrally selective RF pre-saturation pulse is applied before the RF excitation pulse to suppress the signal of the unwanted chemical species. The chemical saturation technique is explained by Harms S E, Flaming D P, Hesley K L, et al. xe2x80x9cMR imaging of the breast with rotating delivery of excitation off resonance; clinical experience with pathologic correlationxe2x80x9d (Radiology 1993; 187:493-501); Haase A, Frahm J. xe2x80x9cMultiple chemical shift selective NMR imaging using stimulated echoesxe2x80x9d (J Magn Reson 1985; 64:94-102); Pauly J M, Nishimura G G, Macovski A, xe2x80x9cMultidimensional selective excitationxe2x80x9d (Proc Soc Magn Reson Med 1988;7:654); Joseph, P M, xe2x80x9cA spin echo chemical shift MR imaging techniquexe2x80x9d (J Comput Assist Tomogr 1985; 9:651-658); and Dumoulin C L, xe2x80x9cA method for chemical-shift-selective imagingxe2x80x9d (Magn Reson Med 1985; 2:583-585). This technique, however, is sensitive to B0 and B1 inhomogeneities, and it also either increases the TR time or reduces the number of imaging slices.
Another method is the Dixon method, which is described in G. Glover et al, xe2x80x9cThree-point Dixon technique for true fat/water decomposition with B0 inhomogeneity correction,xe2x80x9d (Magn. Reson. Med. 1995; 34:120-124). Although it provides water-only and fat-only images simultaneously and is less susceptible to the Bo inhomogeneity, it increases imaging time in gradient echo sequences. A modified Dixon method has been developed to reduce imaging time, as described by Lethimonnier F, Franconi F, Akoka S. xe2x80x9cThree-point Dixon method with a MISSTEC sequence.xe2x80x9d (Magnetic Resonance Materials In Physics, Biology and Medicine 1997; 5(4): 285-288), but there is an accompanying loss in signal-to-noise ratio (SNR).
Recently, an echoplanar spectroscopic imaging technique that produces water, fat and water-plus-fat images without in-plane chemical-shift artifacts is described by Sarkar S, Heberlein K, Metzger G J, Zhang X D, Hu X P, xe2x80x9cApplications of high-resolution echoplanar spectroscopic imaging for structural imaging.xe2x80x9d (J Magn Reson Imaging 1999;10:1-7). However, this technique suffers from poor SNR and cannot be used for high resolution imaging in clinical settings.
Another approach for simultaneous water and fat imaging uses spatial-spectral excitation with alternating water and fat acquisition schemes, and has been implemented on 2-D gradient echo (GRE) sequences, as described by Meyer C H, Pauly J M, Macovski A, Nishimura D G. xe2x80x9cSimultaneous Spatial and Spectral Selective Excitation.xe2x80x9d (Magn Reson Med 1990; 15:287-304). Spatial-spectral excitation has also been applied to 2-D SE imaging, as described by Schick F. xe2x80x9cSimultaneous highly selective MR water and fat imaging using a simple new type of spectral-spatial excitation.xe2x80x9d (Magn Reson Med 1998; 40:194-202) and has been found to provide better fat suppression than the conventional pre-saturation technique. Schick teaches 2D gradient echo imaging in which (water+fat) and (waterxe2x88x92fat) signals are acquired. That technique requires at least two acquisitions and thus provides no saving in time compared to conventional fat and non-fat suppressed imaging. In-plane chemical shift correction by processing is not performed, and through-plane chemical shift misregistration cannot be corrected. Meyer et al teaches 2D gradient echo imaging in which water and fat signals are acquired alternately every TR/2 period. In-plane chemical shift is not mentioned. The maximum number of imaging slices is reduced relative to the regular 2D GRE sequence.
Recently, the present inventors developed a 3-D GRE pulse sequence that produced water images and fat images in a single acquisition time using spatial-spectral excitation. That study also provided a correction for the chemical shift artifacts that may hinder the evaluation of diseases such as osteonecrosis.
It is an object of the present invention to address all of the above issues.
It is another object of the invention to have water-only and fat-only SE images without interference of each other
It is still another object of the invention to acquire water-only and fat-only images during the same acquisition time, to acquire water-only and fat-only images, and to produce water-plus-fat images without chemical-shift artifact.
It is yet another object of the invention to develop a dual-echo spin echo (SE) sequence that simultaneously provides PDW water-only, PDW fat-only, PDW water-plus-fat images without chemical-shift artifact, and T2W water-only images in a single imaging time.
To achieve the above and other objects, the present invention is directed to a technique in a 2-D variable-echo dual-echo SE sequence which will be referred to as interleaved water and fat dual-echo spin echo imaging without chemical shift (IWFSEC). The technique can be implemented on commonly available equipment such as a 1.5 Tesla clinical scanner. By making efficient use of the timing between the first and second echoes, IWFSEC enables the simultaneous acquisition of PDW water-only, PDW fat-only and T2W water-only images in a single imaging time, while maintaining the same maximum number of imaging slices compared to that of the regular dual-echo SE sequence. Since the water and fat signals are excited and acquired at their individual resonant frequencies separately, the chemical shift between water and fat is removed automatically. This enables us to combine the PDW water-only and fat-only images to form water-plus-fat images that are free of in-plane and through-plane misregistration due to chemical shift.
The pulse sequence of the present invention provides 2D SE water-only and fat-only images without interference from each other. It acquires water-only and fat-only images during the same data acquisition time and produces water-plus-fat images with perfect local registration, eliminating the chemical shift artifact, which none of the sequences of the prior art has been able to provide so far.
One aspect of the present invention is in the optimized use of the time between the first and second water echoes to excite and acquire the fat signal. This is not a trivial achievement, since regularly the water and fat signals interfere with each other, so that the images of both species are affected. However, by using tailored spatial-spectral excitations as well as dephasing and rephasing field gradients, the present invention effectively uncouples the water and fat signals. The present invention allows both sets of water and fat images to be obtained in a single acquisition time and saves half of the scan time over conventional fat suppressed and non-fat suppressed imaging. The present invention further provides a technique for intrinsically eliminating the in-plane and through-plane chemical shift misregistration between the water and fat images. The water and fat images can then be combined to form water-plus-fat images free of chemical shift artifacts in both in-plane and through-plane directions.
The present invention thereby provides the following advantages over Schick and over Meyer et al. The present invention provides a 2D dual-echo spin-echo imaging technique for acquiring water-only and fat-only signals within a single acquisition time, thereby saving half of the scan time. Both in-plane and through-plane misregistration caused by chemical shift are intrinsically eliminated. Fat signals are acquired between the first and second water signals during every TR period. The same maximum number of imaging slices as in the regular 2D SE sequence is maintained.
The present technique is taught in Kwok et al, xe2x80x9cInterleaved Water and Fat Dual-Echo Spin Echo Imaging with Intrinsic Chemical-Shift Elimination,xe2x80x9d Journal of Magnetic Resonance Imaging, 13:318-323 (2001), in which it is called interleaved water and fat dual-echo spin echo imaging without chemical shift (IWFSEC). The disclosure of that article is hereby incorporated by reference into the present disclosure.